When you originally signed up for pinterest, did you sign up with your Facebook account? That was my problem. I thought I had signed in, but I actually wasn't. I've been bugging the dev on twitter to fix the app, but no response.

I emailed them seeing if there were plans for an official app and this is the response I got

" Hi Jenna,

Unfortunately, I cannot provide a timeline in regards to developing an app for Windows. We know this is a highly requested app, and something our team will consider. Thanks for your interest and for using Pinterest!